Melt the butter, water and chopped dark chocolate in a saucepan over low heat, stirring continuously until the mixture is smooth.
Remove from heat and whisk in the caster sugar until glossy.
Add the eggs one at a time, stirring well after each addition, then stir in the vanilla extract.
Sift in the flour, cocoa powder, baking powder and salt, folding gently until just combined.
Cookie Dough
Cream together the softened butter, caster sugar and light brown sugar until pale and fluffy, about 5 minutes.
Beat in the egg yolk and vanilla extract until incorporated.
Stir in the flour, baking powder and salt until just combined, then fold in the chocolate chips.
Assembly & Baking
Preheat the oven to 180°C fan (200°C/400°F)/gas 6 and line a 20cm square tin with baking paper.
Dollop the brownie batter into the tin, smoothing it to the edges.
Top with spoonfuls of cookie dough and use a skewer to swirl the two batters together.
Bake for 25–30 minutes until the edges are set but the centre still wobbles.
Allow to cool completely in the tin before slicing into squares.
Notes
If the cookie dough is too sticky to dollop, chill it in the fridge for 10 minutes.For extra gloss on the top, gently tap the filled tin on the counter before baking to release any air bubbles.